1. Field of the Invention
The present invention relates to energy storage devices. More specifically, the present invention relates to supercapacitors comprising carbon nanotube electrodes.
2. Related Art
Rapid advances in nanoscience and nanotechnology have led to the ground-breaking discovery of carbon nanotubes. Carbon nanotubes are a new form of carbon-based material and exhibit many unique properties, such as extraordinary strength, and excellent electrical and heat conductivity, thereby making them ideal candidates for many important applications.
In particular, carbon nanotubes are attractive electrode materials for fabricating electrochemical energy storage devices because of their high electrical conductivity, chemical stability, low mass density and large accessible surface area. Consequently, a wide range of potential applications for electrochemical energy storage systems based on carbon nanotubes have been proposed recently. For example, carbon nanotubes have been formed into electrodes for Lithium-Ion secondary batteries, for hydrogen storage in fuel cells, and for supercapacitors.
As unique energy storage devices, supercapacitors can provide significant amount of energy in a short period of time, i.e., high power density. Practically, the power density of supercapacitors can be many times larger than that of secondary batteries, making them indispensable for surge-power delivery, such as in hybrid-electric and fuel-cell powered vehicles.
Typically, a supercapacitor comprises two electrodes separated by an electrolyte, wherein each electrode further comprises a thin layer of active material (e.g. carbon nanotubes) prepared on a metal substrate (current collector). Theoretically, the maximum power density of a supercapacitor is given by Pmax=Vi2/4R, where Vi is a initial voltage, and R is an equivalent series resistance (ESR) of the supercapacitor which includes both the internal resistance in the layer of active material and the contact resistance between the layer of active material and the current collector. Accordingly, reducing the ESR is the key to attain high power density.
There have been several successful demonstrations of high power density supercapacitors based on carbon nanotubes. So far, an 8 kW/kg power density and a 20 kW/kg power density have been reported using multi-walled carbon nanotubes and single-walled carbon nanotubes, respectively. Unfortunately, single-walled carbon nanotubes are significantly more expensive than multi-walled carbon nanotubes. Furthermore, use of a binder is often required to hold the carbon nanotubes together in those supercapacitors which typically results in an increased contact resistance between the layer of active material and the current collector. Additionally, those supercapacitors which can achieve 20 kW/kg power density typically require subsequent high temperature treatment (1000° C.) after the carbon nanotube electrodes are constructed.
Hence, what is needed is a method for fabricating a high power density carbon nanotube supercapacitor without the above-described problems.